1. Field of the Invention
The present invention relates to an optical head device and an optical information reproducing device that are used for reproducing information recorded on a magneto-optical recording medium or an optical information recording medium such as an optical disc or an optical card. Particularly, the present invention relates to an optical head device and an optical information reproducing device that are suitable for reproducing information recorded on an optical information recording medium having a plurality of information layers (e.g., a multilayer optical disc or a multilayer optical card).
2. Related Background Art
In recent years, in order to increase the recording capacity of an optical information recording medium such as an optical disc, investigations have been made for enhancing the recording density. In order to realize such an enhancement of the recording density, it has been considered to make a spot size minute, for example, and more specifically it has been considered to shorten a wavelength of a light source and to increase a numerical aperture (NA) of an objective lens. Although the wavelength of the light source is 650 nm and the NA of the objective lens is 0.6 in the present digital versatile discs.(DVD), to employ an optical system with the light source wavelength of 405 nm and the NA of the objective lens of 0.85 is suggested for next-generation optical discs. Furthermore, as an optical disc with a still increased capacity, a multilayer optical disc is now being developed, in which a plurality of information layers are superimposed in a thickness direction of the optical disc with a predetermined space therebetween.
As for the multilayer optical disc, there is a problem of interlayer crosstalk, in which, during reproduction of recorded information, a signal from another information layer is leaked in as well as a signal of a target information layer to be reproduced (hereinafter referred to as a “reproduction information layer”). Since the interlayer crosstalk decreases with an increase in distance between the reproduction information layer and other information layers, a leak of a signal from an information layer adjacent to the reproduction information layer plays a dominant role for the interlayer crosstalk. Therefore, it is possible to decrease the interlayer crosstalk to a practically sufficient low level by increasing a space between the reproduction information layer and the adjacent information layer.
However, when the space between the layers is large, a spherical aberration due to a difference in thickness of a base material (herein, the base material for each information layer generically refers to a portion located on an incident side of laser light relative to the information layer of interest in the multilayer optical disc) varies significantly between the respective information layers of the multilayer optical disc. Therefore, it is difficult to increase the number of information layers beyond a correctable range of a spherical aberration correction means. For instance, in an optical system with a light source wavelength of 405 nm and a NA of an objective lens of 0.85, about 0.01λ of spherical aberration occurs per 1 μm of an error of a base material thickness. When spaces between the respective layers are made 20 μm, it is possible to decrease the interlayer crosstalk to a sufficient level. However, in the case where the aberration correctable range of the spherical aberration correction system is 100 μm, six information layers, at most, can be laminated. For that reason, it is not appropriate that the interlayer crosstalk is suppressed by increasing a space between layers for the purpose of a still larger capacity. Therefore, the interlayer crosstalk has to be suppressed without increasing the space between the layers.
To cope with the above-stated problem, a method is proposed in which a signal light receiving area for an adjacent information layer is provided on the periphery of a signal light receiving area for a reproduction information layer and signals from the respective areas are calculated so as to cancel the interlayer crosstalk (See JP 2002-319177 A, for example). Furthermore, another method is proposed in which astigmatism is imparted to light reflected from a multilayer optical disc and a light receiving element is arranged so that light reflected from a reproduction information layer forms a circle of least confusion and light reflected from an adjacent information layer forms a focal line. In this way, a light receiving area at the focal line portion is separated optically from a light receiving area in the circle of least confusion, whereby information only from the reproduction information layer is reproduced (See JP H11(1999)-242824 A, for example). JP H11(1999)-242824 A further suggests that the light reflected from the reproduction information layer and the light reflected from the adjacent information layer are separated by using a hologram element and a signal obtained from each reflected light is calculated to cancel the interlayer crosstalk.
Since an amplitude and a frequency of the interlayer crosstalk depend on a space between a reproduction information layer and an adjacent information layer, a signal of the adjacent information layer needs to be deducted from a signal of the reproduction information layer in accordance with the space between the layers so as to remove an interlayer crosstalk component with high accuracy.
However, according to the method disclosed in JP 2002-319177 A, signals of the information layers other than the reproduction information layer are detected collectively, which are then deducted from the signal of the reproduction information layer. That is to say, a signal of the adjacent information layer disposed frontward and closer to an objective lens of an optical head device than the reproduction information layer and a signal of the other adjacent information layer disposed backward at a more distant position are not separated, and a signal for each of the adjacent information layers cannot be detected. Therefore it is difficult to cancel the interlayer crosstalk optimally for each of the adjacent information layers. For instance, in the case where a space between the frontward adjacent information layer and the reproduction information layer and a space between the backward adjacent information layer and the reproduction information layer are different due to a manufacturing error and the like, the crosstalk component cannot be removed so as to reflect the respective spaces between the layers, thus making it difficult to obtain good reproduction signals. Meanwhile, according to the method disclosed in JP H11(1999)-242824 A, each of the signals of the frontward adjacent information layer and the backward adjacent information layer is separated from the signal of the reproduction information layer. However, when the respective signals are calculated, consideration is only given to an area of the light receiving area and not to a space between the layers.